The development of tunable solid state lasers based on the Cr4+-ion started in 1988 with forsterite, Cr:Mg2SiO4 [V. Petricevic, S. K. Gayen and R. R. Alfano, Appl. Phys. Letters 53 (1988) 2590]. It was extended to other crystalline media, such as Cr4+-doped Y3Al5O12 [A. P. Shkadarevich, in: OSA Proceedings on Tunable Solid State Lasers, Ed. M. L. Shand and H. P. Jenssen (Optical Society of America, Washington, D.C., 1989), Vol. 5, pp. 60-65], Y2SiO5 [J. Koetke, S. Kuck, K. Petermann, G. Huber, G. Gerullo, M. Danailov, V. Magni, L. F. Qian, and O. Svelto, Opt.Commun. 101 (1993) 195], Y3ScxAl5-xO12 [S. Kuck, K. Peterman, U. Pohlmann, U. Schonhoff, and G. Huber, Appl.Phys. B58, (1994) 153]. These latter materials retain chromium dopant in crystalline structure in other valence states, which act as traps and reduce the concentration of Cr4+ lasing ions.
The Cr4+-ions in tetrahedral coordination is useful for realization of room temperature tunable solid state laser operation in the spectral range from 1.1 to 2 μm. Unfortunately, for all the materials listed above, the concentration of Cr4+ ions in the crystalline structure was lower than 0.1%, and chromium in other valence states (for example, Cr3+ and Cr2+) was present in those crystals. This led to complicated spectroscopic properties of the materials. As a result, the active media length in the laser devices was too high for at least some applications. This took these crystals out of consideration for small micro-laser development, where thickness of laser element generally does not exceed a few mm in length.